The present invention relates generally to laser eye surgery methods and systems. More specifically, the present invention relates to methods and systems for stabilizing an amount of laser energy delivered to a target.
Known laser eye procedures generally employ an ultraviolet or infrared laser to remove a microscopic layer of stromal tissue from the cornea of the eye to alter the refractive characteristics of the eye. The laser removes a selected shape of the corneal tissue, often to correct refractive errors of the eye. Ultraviolet laser ablation results in photo-decomposition of the corneal tissue, but generally does not cause significant thermal damage to adjacent and underlying tissues of the eye. The irradiated molecules are broken into smaller volatile fragments photochemically, directly breaking the intermolecular bonds.
Laser ablation procedures can remove the targeted stroma of the cornea to change the cornea's contour for varying purposes, such as for correcting myopia, hyperopia, astigmatism, and the like. Control over the distribution of ablation energy across the cornea may be provided by a variety of systems and methods, including the use of ablatable masks, fixed and moveable apertures, controlled scanning systems, eye movement tracking mechanisms, and the like. In known systems, the laser beam often comprises a series of discrete pulses of laser light energy, with the total shape and amount of tissue removed being determined by the shape, size, location, and/or number of a pattern of laser energy pulses impinging on the cornea. A variety of algorithms may be used to calculate the pattern of laser pulses used to reshape the cornea so as to correct a refractive error of the eye. Known systems make use of a variety of forms of lasers and/or laser energy to effect the correction, including infrared lasers, ultraviolet lasers, femtosecond lasers, frequency multiplied solid-state lasers, and the like. Known corneal correction treatment methods have generally been successful in correcting standard vision errors, such as myopia, hyperopia, astigmatism, and the like. By customizing an ablation pattern based on wavefront measurements, it may be possible to correct minor aberrations to reliably and repeatedly provide visual acuity greater than 20/20.
When laser energy is delivered from a laser energy generating device to a target, as in a laser eye surgery procedure, the energy (typically in the form of a laser beam) passes along a delivery path. The laser beam typically follows a path that proceeds through a series of lenses, mirrors and/or other optical elements to focus and/or direct the beam before it arrives at a patient's eye. As laser energy passes along such a delivery path, it typically causes one or more substances to accumulate. The most prevalent and significant substance that accumulates along a UV laser beam delivery path due to passage of the laser beam is ozone. The laser beam creates ozone when it passes through oxygen along the delivery path. Subsequent pulses of the laser beam are then impeded by the presence of ozone along the path, resulting in a reduced amount of energy arriving at the patient's eye with each subsequent pulse. The same reduction occurs over time with a constant wave laser. As ozone continues to accumulate, the laser energy arriving at the eye continues to decrease.
One objective in laser eye surgery is to deliver approximately the same amount of laser energy to the eye with each pulse of the laser, or if constant wave laser energy is used, to deliver a constant amount of energy to the eye over time. Currently available systems and techniques, however, do not account for accumulation of substances such as ozone along the laser beam delivery path, and thus do not provide a constant or stabilized amount of delivered laser energy to the eye. Stabilized delivered laser energy would enhance laser eye surgery by providing a desired amount of energy to an eye over multiple laser pulses and over time to allow for a more precise and accurate laser eye surgery procedure.
Therefore, it would be desirable to provide methods and systems for stabilizing an amount of laser energy delivered to an eye during a laser eye surgery procedure. Ideally, such methods and systems compensate for impedance of a laser beam caused by accumulation of substances, such as ozone, along the laser beam delivery path. Also ideally, such methods and systems could be used to calibrate a laser generating device before performing any laser eye surgery procedures and would not require frequent recalibrations. At least some of these objectives will be met by the present invention.